This is the boss. No calculator — instead you get what a surveyor actually had for two centuries: a table on the paper. $\cos 60^\circ = 0.50$, $\sin 60^\circ = 0.87$, $\sin 30^\circ = 0.50$, $\sin 90^\circ = 1.00$. Every draw is pinned so the arithmetic fits in your head.

One lot, split by a diagonal into two triangles, and every answer feeds the next: the cosine law closes the diagonal, the diagonal feeds the sine law, and both feed the two areas that add to the total. Write each line down the page as you go — a slip on line one walks all the way to the fence.

Everything from the chapter rides in it. 5★ under par here is the chapter crown. Chain up.